This invention relates to fine spherical polymer particles having a uniform particle size distribution and containing a prepolymer and if desired, an inorganic pigment and/or black coloring agent and a resin liquid at normal temperature, which are obtained by effecting polymerization in a stable suspension system free of coalescence of particles and to a process for preparing the same.
In conventional methods for conducting a suspension polymerization in water, a suspending agent is used to prevent polymer particles from coalescing during the progress of polymerization.
Generally, the suspending agents are classified broadly to a water-soluble high molecular substance and fine powders of difficultly soluble inorganic compound, the former being gelatine, starch, polyvinyl alcohol and others, the latter being difficultly soluble salts such as BaSO.sub.4, CaSO.sub.4, BaCO.sub.3, CaCO.sub.3, MgCO.sub.3 and Ca.sub.3 (PO.sub.4).sub.2, inorganic high molecules such as talc, clay, silicic acid and diatomic earth and powdery metal oxides.
The suspension polymerization method for obtaining polymer particles containing an inorganic pigment or coloring agent is disclosed in, for example, U.S. Pat. No. 3,634,251 and Japanese Patent Publication No. 10231/61, in which the suspending agent used is adsorbed to polymer particles or dispersed between particles.
In addition to the selection of suspending agent, stirring is an important requirement for suspension polymerization, according to which particle size and polymerization stability are determined. Stirring at a low speed brings about gelation so that polymer particles cannot be obtained. On the contrary stirring at a high speed results in formation of polymer particles with a small size, however air penetrates into a polymerization vessel through a stirring apparatus so that the yield and degree of polymerization are reduced due to the hindrance effect of oxygen on reactive monomer radicals.
Connected with this is a problem of liquid ratio (polymerization monomers: water), which is usually in the range of 1:5 to 1:8. If water is little and monomers are much, the polymerization reaction product is allowed to gel and the particle size distribution is broad in general.
It is very difficult to obtain polymer particles of less than 30.mu. in size on a commercial scale by conventional suspension polymerization methods. Since polymer particles become more sticky due to increasing viscosity during the progress of polymerization, coalescence of particles is unavoidable even under stirring so that particle sizes become large or gelation occurs. For avoiding such phenomena other additives are used in addition to the suspending agent. There are used a method of adding glycols or glycerine thereby to prevent particles from coalescing or a method of adding electrolytes such as NaCl or Na.sub.2 SO.sub.4 so as to increase interfacial tension between water and particles. However, satisfactory effects have not been obtained yet.